The invention relates to a circuit arrangement for driving a current-controlled component.
Current-controlled components are, for example, lasers and light-emitting diodes used as transmitting elements in optical message transmission comprising optical fibers and a transmission medium. Therein, the light intensity of the laser or of the light-emitting diode is modulated by the input signal. In the digital optical message transmission the transmission of light corresponds to, for example, a logic 1, whereas a logic 0 is present when no light is transmitted.
In The Bell Systems, Technical Journal, Vol. 57, No. 6, July-August 1978, a laser transmitter for optical transmission systems is disclosed on pages 1823 to 1833 in the article "GaAlAs Laser Transmitter for Lightwave Transmission Systems" by Shumate, Chen and Dorman. FIG. 2 on page 1826 shows a circuit arrangement for driving a laser, which is here shown in FIG. 1. The input signal is applied via a transfer stage comprising a transistor Q.sub.3 to one input of a differential amplifier which comprises two transistors Q.sub.1 and Q.sub.2. The other input of the differential amplifier is connected to a reference voltage. A resistor having a value of 10.OMEGA. is included in the collector lead of the transistor Q.sub.1, while the laser is arranged in the collector-lead of the transistor Q.sub.2. The emitters of the two transistors Q.sub.1 and Q.sub.2 are connected to one output of a current source. When the laser is to transmit light, the transistor Q.sub.2 is rendered conductive, whereas the transistor Q.sub.1 is cut off, so that the current source is connected to the laser via the emitter-collector path of the transistor Q.sub.2. If no light is to be transmitted, that is to say there is a pulse pause, the transistor Q.sub.2 is cut-off, whereas transistor Q.sub.1 is rendered conductive, so that the current source is not operated in the no-load mode, but the current circuit is closed via the emitter-collector path of the transistor Q.sub.1 and the 10 .OMEGA.-resistor.
This prior art circuit arrangement has the disadvantage that it has a comparatively high power dissipation, as during the pulse pauses the current produced by the current source does not flow through the laser, but through the 106 .OMEGA.-resistor, in which ohmic losses occur during periods of current flow through the resistor.